OmniBridge and OmniBridge UI is experimental software in Beta, use at your own risk. The OmniBridge UI can be accessed here https://omni.xdaichain.com
More information is available on the xDai instance here.
Instructions on Using the UI
Another option for a token owner (without programming knowledge) is to use solUI.
Here is an example application where users can transfer JOON tokens from the Ethereum Mainnet to the xDai chain and back.